In both the field of development of radiation counter measures for large population administration as a radiation counter measure and in the field of development of normal tissue radiation protectors for clinical radiotherapy, interest in new small molecule modifiers of irradiation induced cellular tissue and organ damage has gained prominence in recent years. Reports of effective small molecule irradiation mitigators have included the GS-nitroxides, triphenylphosphonium conjugated Imidazole Fatty Acids, phospho-inositol-3 kinase inhibitors, and a variety of other small molecules which inhibit ionizing radiation induced apoptosis. Delivery of these small molecules at 24 hours or later after total body irradiation has proven effective in animal models of total body irradiation and in some cases, as with GS-nitroxides, has been effective in multiple organ specific administration protocols for protection of the esophagus and skin from ionizing irradiation damage.
A challenge for the development of small molecule irradiation mitigators has been design and implementation of a non-toxic and reliable delivery system. Relative insolubility of many new small molecule radiation mitigators has required administration by intravenous, intra-peritoneal, or other systemic delivery systems. Delivery formulations have required liposomal or other solvent systems that have been unsuitable for oral administration.
In addition, modern drug development relies on high-throughput screening assays. Often, these trials use compound libraries stored in solution for periods of several months to as long as three years. DMSO 1 has been used as the storage solvent of choice, but problems, including compound degradation and precipitation, are frequently encountered. In one case study, qualitative compound precipitation was observed in 26% of test plates. Systematic studies of compound degradation in DMSO have indicated that approximately 50% of samples degraded over a period of 12 months when stored in anhydrous DMSO at ambient temperature. Compound storage problems are augmented by low hydrophilicity, since a large portion of screening libraries is composed of compounds designed for enhanced membrane permeability. The trend toward lipophilic, higher molecular weight compounds results in libraries of materials with lower intrinsic aqueous solubilities. Current estimates state that 30-50% of compounds in screening libraries have aqueous solubilities of less than 10 μM. These lipophilic molecules are more likely to precipitate from DMSO stock solutions, leading to erroneously low assay concentrations when using the DMSO stock for sample preparation. Additionally, poor aqueous solubility causes precipitation from aqueous media after dilution of DMSO stock solutions. When compound concentrations in assay media fall below calculated concentrations, flawed conclusions regarding toxicity, efficacy, or structure-activity relationships are drawn.
Aqueous dissolution of problematic compounds can be enhanced by salt formation, or chemical modification of the substrate (formation of pro-drugs). If these methods are not applicable, complexing agents or cosolvents can be added to aid in dissolution. Some examples include cyclodextrins, dendrimers, low molecular weight PEG's (polyethylene glycols, e.g., PEG 400), and solvents such as glycerin and NMP (A-methyl pyrrolidone). Other solubilizing agents have designs based on DMSO; one example is a polymeric sulfoxide derived from poly-L-methionine 2 (FIG. 1).